428 T. H. MORGAN. 



the arrangement of the septa was found to be normal ; that is 

 to say, the septum has met normally across the mid-dorsal line, 

 but the external lines have failed to do so. 



In fig. 72, A B, a surface spiral is drawn, and in b the arrange- 

 ment of the septa beneath is shown. The septa shows an 

 abnormal arrangement, but this does not correspond to the 

 surface lines : it is more of an approach to the normal. 



Fig. 73 is from a compound metamere but in which the 

 septa form a simple spiral making a little more than one turn 

 of the body. 



It is not always easy to picture to one's self the relation 

 existing between septal and surface lines when the two do 

 not correspond. In the simpler cases it is easily understood. 

 When two spirals are formed of different lengths, it is due to 

 the fact that the septa sometimes unite with one another 

 beneath the surface before the surface spiral is brought to an 

 end. That is, if both start together one may continue for a 

 longer time than the other. We must conclude, then, that 

 while the two usually vary together, yet they may vary inde- 

 pendently. 1 have dissected a far greater number of worms 

 than recorded above, and from that number have selected 

 those given as the most interesting examples to illustrate the 

 main points that concern us at present. Many other rela- 

 tions have suggested themselves, but I have only wished to go 

 into the subject as far as concerns the matter in hand. 



VII. Study of Polych^tous Annelids and Leeches. 



A large number of species of polychaetous Annelids have 

 been examined, and many modifications in the arrangement 

 of the metameres have been found in them. From this number 

 I have chosen a very few for description, since a large number 

 of cases have been given in the papers by Cori and Buchanan. 



The modifications that are figured here are all taken from 

 specimens of Amphinome. 



In fig. 82, A B, we find above b a compound metamere with 

 the split on the left side. In the ventral side of the worm, a, 

 we find the left anterior half of the compound metamere 



